Ecology of Population

A population is the total number of organisms of the same species living in a specific area at a given time. In an ecosystem, a community comprises multiple populations of different species.

Characteristics of a Population

1. Population Size: The total number of individuals of the same species in a specific area. Larger populations are more resilient to threats like disease and natural disasters.
2. Population Frequency: The number of times a species appears in a specific area or habitat.
3. Population Density: The number of individuals per unit area or volume of a habitat.
4. Population Distribution: The spatial arrangement of individuals in a population. This can be clumped, random, or uniform, depending on factors like resources and social behavior.
5. Population Growth: The change in population size over time, influenced by birth rates, death rates, immigration, and emigration. Growth can be exponential or logistic, depending on resource availability.

Factors Affecting Population Size

• Natality (Birth Rate): Higher birth rates increase population size, while lower rates decrease it.
• Mortality (Death Rate): Higher death rates reduce population size, while lower rates increase it.
• Immigration: The movement of individuals into a habitat increases population size.
• Emigration: The movement of individuals out of a habitat decreases population size.
• Seasonal Climate Changes: Favorable conditions can attract organisms, while unfavorable ones cause migration.
• Breeding Seasons: Some species migrate during breeding, temporarily reducing population size in their original habitat.
• Food Availability: Abundant food increases population size, while scarcity reduces it.
• Heat and Water: High temperatures or lack of water can decrease population size.
• Nutrients: Plants require soil nutrients for growth. Deficiencies can lead to poor crop yield and stunted growth.
• Light: Plants depend on light for photosynthesis. Low light intensity weakens plants and reduces food production.

Dynamic Equilibrium

In a stable ecosystem, population sizes fluctuate around the carrying capacity of their habitats. While population numbers remain relatively constant, new individuals are added as old ones are removed, maintaining a balance called dynamic equilibrium.

This balance is influenced by:

1. Density-Independent Factors: Factors like weather changes, natural disasters, and pollutants affect population size regardless of density.
2. Density-Dependent Factors: Factors like predation, competition, parasitism, and disease are more significant in dense populations, often slowing population growth.

Methods of Studying Populations

To study populations, follow these steps:

1. Select a habitat.
2. Choose a sampling method.
3. Identify the species in the habitat.
4. Count and record the organisms.
5. Repeat the study at different times.

Sampling methods include:

• Quadrat Sampling: A quadrat is made of a square or rectangular piece of wire, wood or metal frame with predetermined area. E.g. the area of a quadrat may be 25cm². A quadrat is used to sample the number of plant species in a habitat. It is not suitable for sampling animals because they move around so much. A quadrat is used by throwing it over the shoulder at random several times and on each landing, the area covered/enclosed by it is observed. The type of plant species and their number within the quadrat are recorded. From the results, the average number of plant per m² is calculated. If the area of the habitat is known, the total number of plants it contains can be estimated.

• Transect method: A marked tape is used in this method. The tape is marked at convenient intervals and then stretched across the area to be studied. The plants encountered at the interval marks are counted and recorded. This procedure is repeated a few times. In this way, a fairly accurate estimate of the number and types of plants in the habitat are obtained. Plants are usually collected in plastic bags and then pressed and dried in a plant press. The dried plants are mounted on stiff paper, fixed in position with masking tape/cellotape and labeled with both the scientific and common names.
• Capture-Recapture: Animals are captured, marked, and released. A second capture is conducted to estimate population size using the formula:
  (Number first captured × Number second captured) ÷ Number recaptured

Family Planning

Family planning involves using birth control methods to control the number and timing of children. These methods include:

1. Natural Methods:
   • Rhythm Method: Avoid intercourse during fertile periods in the menstrual cycle. This method is unreliable due to varying cycle lengths.
   • Withdrawal: The penis is withdrawn before ejaculation. This is unreliable as sperm may enter the vagina before ejaculation.
2. Mechanical Methods:
   • Condom: A sheath covering the penis to prevent sperm from entering the vagina. It also protects against sexually transmitted diseases.
   • Diaphragm: A rubber cap placed over the cervix to block sperm from entering the uterus. Often used with spermicides.
   • Intrauterine Device (IUD): A device placed in the uterus to prevent implantation of fertilized eggs.
3. Chemical Methods:
   • Spermicides: Chemicals in creams or tablets that kill sperm. Used with diaphragms or condoms for added protection.
   • Contraceptive Pills: Pills containing hormones to prevent ovulation. Effective if taken consistently.
4. Surgical Methods:
   • Vasectomy: In males, the sperm ducts are cut and tied to prevent sperm from being released.
   • Tubal Ligation: In females, the fallopian tubes are tied to block sperm from reaching eggs. This method is irreversible.